For the past 150 years or so, one of the most efficient means to convey information about the world we live in has been through the use of two dimensional images. These images have been recorded and displayed using chemical or more recently with electronic techniques, and have been very effective at capturing and conveying the light intensity, color, and x and y coordinates of targets within a scene. Since these images record the two dimensional projection of a 3D world onto an imaging plane, all quantitative depth information is lost. Only subtle cues such as shading and the mind's a priori knowledge of the scene allow the human observer to qualitatively extract depth cues from these two dimensional projections. Clearly, even these qualitative depth cues can fail the observer in situations where the target is completely unknown, or if depth ambiguities in the 2D projection exist which can easily fool the human observer.
Because of the inherent limitations of 2D technology, making the transition from 2D to 3D image capture and display is extremely desirable. In terms of 3D capture, any endeavor requiring the quantitative measurement of a target's dimensions would benefit from a sensor system that could also measure depth. Some obvious applications include quality control of parts on an assembly line, facial recognition for security, autonomous vehicle navigation, measurement and registration of tissues during medical procedures, and 3D model capture for the movie and entertainment industries. Indeed, it is difficult to imagine any imaging application where the added capability to measure a target's depth along with its color, intensity, and its x and y position would not be welcome.
In terms of transitioning to 3D display technologies, only the latter can take advantage of the human user's “built in” stereoscopic vision system to further enhance the sensation of depth. Just as the use of fully rendered and shaded 2D projection graphics was a huge improvement over the previous use of point clouds, wireframe drawings or flat color projections, the jump to full fledged 3D display represents a huge improvement in how the user can perceive and manipulate target information. Indeed, the final addition of the stereoscopic depth cue should allow for the creation of near perfect virtual optical input; so perfect that the user has difficulty in differentiating between a real scene and its recorded version.
In order to overcome the inherent limitations of standard 2D imaging techniques, research into technologies that can capture and display the full three dimensional information in a scene has been very active. A survey of current technology suggests that image recording and display technology is in a transition stage from two dimensional to three dimensional techniques. In the case of 3D recording techniques, one of the oldest approaches to capturing depth information has been through the use of a pair of 2D recording devices whose relative orientation is known. Much like the optical system of predator animals in nature, this depth from stereo approach is a triangulation technique that relies on the difference or disparity between the recorded position of each target point on the two imaging sensors. For each target feature, the magnitude of this disparity is directly related to that feature's distance from the imaging system. Some of this technique's shortcomings include occlusion (where a target feature is visible to one sensor but not to the other), the need to constantly calibrate the system due to camera alignment difficulties, high cost due to the use of multiple camera/lens systems, and high computational cost due to large target feature disparities between one sensor and the other.
Laser scanning systems represent another approach that utilizes triangulation in order to capture the depth information in a scene. In a typical system, a laser sheet is swept across the target object and the position of the resultant line (which is the laser sheet's projection onto the target surface) is recorded by a camera. Just like in stereo camera systems, the relative position and orientation of the recording camera and the laser projector are known, so that the calculation of the laser line's position uniquely determines the distance from the sensor of target features along that line. Though laser scanners are amongst the most accurate 3D imaging systems available, some of this technique's inherent shortcomings include high cost, size and complexity, and scanning times on the order of seconds or minutes.
Time of flight range scanners, meanwhile, measure the time taken by a pulsed laser beam to bounce off a target and return to its source. These systems are often used in surveying applications as their operating range reaches up to 100 meters. Their positional accuracy is typically about 6 mm at 50 m distances, but their disadvantages include their complexity and expense, as well as prolonged operation time required to scan a full target scene.
Much progress has also been made in the development of devices that can use and display 3D data. For decades, computer graphics libraries have existed that allow the display and manipulation of shaded, photo realistic 2D projections of 3D objects on standard 2D displays. The ability to handle 3D data and display realistic 2D projections of that data has been useful in fields ranging from computer aided design (CAD) to entertainment applications like movies and video games. More recently, devices that seek to give the user an actual sensation of the target's depth (as opposed to just displaying the 2D projection of a 3D object) have become more common in the marketplace. For example, wearable display glasses that transmit the left and right stereoscopic views to the user's corresponding eyes have been in use for some time and are quite good at creating a virtual reality effect. Computer screens with modified LCD screens are also appearing in the marketplace with some success. These create the impression of depth by having part of the LCD screen project the left image so that it is only visible to the user's left eye, and by having the other part of the LCD screen project the right image so that it is only visible to the user's right eye. Holography based technologies are also being developed that allow multiple users to achieve the same depth sensations from differing vantage points. A similar effect can also be obtained by projecting different 2D views of the 3D data on a rapidly rotating screen.
Technologies to manipulate and display 3D datasets (either in full 3D fashion or simply through 2D display of photorealistic projections) are more widely diffused than 3D recording devices. This gap between 3D input and output technologies will need to be filled by improvements in current measurement devices, as well as by the development of entirely new approaches to the problem. One such approach may well be 3D surface imaging using active wavefront sampling.